This invention relates to hermetic coatings for optical fibers.
Optical fibers are conventionally provided with abrasion resistant coatings of resins such as silicone, polyurethane acrylate, or the like. Such coatings are typically applied on-line as the optical fibers are drawn so that surface abrasion is avoided during the fiber pulling process. There presently exist a variety of coatings which protect a fiber from abrasion but not from corrosion or hydrogen diffusion.
Various chemicals, including water, can react with a fiber, damaging its optical properties and weakening its mechanical strength and static fatigue resistance. Microcracks in a fiber surface present regions susceptible to chemical attack, especially when the fiber is under stress. Fiber stress tends to open a crack, thereby focusing the strain onto the chemical bonds at the tip of the crack. These strained bonds are more easily chemically attacked, thereby enabling corrosion to extend such microcracks. Growth of microcracks weakens the strength of a fiber producing static fatigue or sudden failure.
The effect of stress corrosion on t.sub.s, the time to failure of an optical fiber under static stress, is determined in part by the crack velocity exponent n. Typically, a fiber having a large value of n also has a large value of t.sub.s under typical values of applied stress; if it doesn't break during a relatively short test period, it is certain to last for a long time under typical use conditions. For a discussion of optical fiber strength and fatigue characteristics, see K. E. Lu et al. "Mechanical and Hydrogen Characteristics of Hermetically Coated Optical Fiber" Optical and Quantum Electronics, vol. 22, (1990) pp. 227-237.
Diffusion of hydrogen into the fiber is detrimental to its optical performance. An attenuation increase may occur after an optical fiber has been installed, thereby creating the possibility that the system will be rendered at least temporarily inoperative.
The following test was employed to determine the permeability of carbon coatings to hydrogen. Fibers having 1 km lengths were placed in 85.degree. C. chambers containing 11 atmospheres of hydrogen for 20 days. Attenuation was measured at the 1242 nm hydrogen absorption band, and the measured attenuation is an indication of whether the carbon coating is an effective hydrogen barrier. A hydrogen barrier factor of 0.2 dB/km, for example, means that the attenuation of the fiber increased by 0.2 dB/km at 1242 nm during the 20 day test. A hydrogen barrier factor of 0.02 to 0.2 dB/km after 20 days is considered good; less than 0.02 dB/km is considered excellent. Without a hydrogen barrier, the hydrogen absorption of a fiber will reach 50 dB/km in less than three days when exposed to 11 atmospheres of hydrogen at 85.degree. C.
Metallic and ceramic coatings have been used with varying degrees of success with respect to the reduction of microcrack degradation. However such coatings are not sufficiently impermeable to hydrogen.
It has been known that carbon coatings can produce water resistant, high strength optical fibers (see U.S. Pat. No. 4,183,621 entitled "Water Resistant High Strength Fibers" issued to Kao et al. on Jan. 15, 1980). For various reasons, initial attempts at depositing carbon on optical fibers were unable to produce coatings that were impermeable to water and/or hydrogen and were unable to produce long lengths of coated fiber. For example, U.S. Pat No. 4,512,629 reports n was determined to be only 30.3 for a 300 Angstrom carbon coating deposited by sputtering and that the value of n was only 8 for the case of a 100 Angstrom carbon coating deposited on-line by chemical vapor deposition. A further disadvantage of prior art coating apparatus was its inability to produce carbon coated optical fiber at draw rates above 1 meter/sec. Various types of prior art coating apparatus are discussed below to illustrate these disadvantages.
FIG. 1 shows a typical prior art apparatus for depositing a coating of pyrolytic carbon on an optical fiber 10. Tractor means (not shown) pulls fiber 10 from the bottom tip of preform 11 which is heated by furnace 12. Included on the draw tower are carbon coating apparatus 13, fiber cooling tube 14 and coating means 15 where a coating of abrasion resistant material is formed on the fiber. A diameter measurement device may be located between the furnace 12 and apparatus 13. A device such as Q meter 16 may be located below apparatus 13 to measure the resistivity of the carbon coating on the draw and thereby provide an indication of coating continuity and thickness. Such contactless devices for electrically measuring the thickness of a coating are disclosed in U.S. Pats. Nos. 4,593,244 and 3,679,968. The resistivity of the coating on ends of the coated fiber can thereafter be measured off-line by a contact apparatus to calibrate the Q meter.
Apparatus 13 comprises a first isolation chamber 19, a reaction chamber 20 and a second isolation chamber 21. Chambers 19 and 20 are connected by a small-diameter opening 22a, and chambers 20 and 21 are connected by a small-diameter opening 22b. Isolation chambers 19 and 21, which isolate reaction chamber 20 from the ambient atmosphere, have small-diameter openings 22c and 22d, respectively. Inert gas flows through inlets 23 and 24 to chambers 19 and 21, respectively, to provide those chambers with a sufficient pressure to restrict the flow of atmospheric air into openings 22c and 22d. As indicated by double-headed arrows 25a and 26a, various types of prior art apparatus are designed such that the reactant gas has been flowed into either pipe 25 or pipe 26, and the reaction products have flowed out of the opposite pipe. In general, reactant gas is flowed toward one side of the fiber. Optical fiber 10 is introduced into apparatus 13 from opening 22c, passed through isolation chamber 19, reaction chamber 20 and the second isolation chamber 21, and it exits the apparatus through opening 22d.
At least some of the heat for the reaction is supplied by the heat of the drawn fiber. In the absence of auxiliary heating means, the fiber temperature in the reaction chamber is dependent upon the fiber diameter, the draw rate and upon 1/L, where L is the distance from the neck-down part of the fiber preform in the furnace. For low draw rates, for example, reaction rate or efficiency can be increased by heating the fiber prior to its entry into the reaction chamber or while it is in the reaction chamber, or by heating the reactant gas before it reaches the reaction zone. Heating coil 27 is illustrative of various types of auxiliary heating means. Techniques which preheat the gas can cause the reaction to occur at a location that is sufficiently remote from the fiber surface that an excessive deposition of carbon builds up on the apparatus. Such carbon buildup can occur at openings 22a and 22b and can therefore abraid and weaken the fiber. The products of such a reaction can flow through exhaust pipe 25 (or 26) and block filters or otherwise restrict flow, thereby unblancing process flow conditions. Also, particles can break away from the apparatus and fall through opening 22d to coater 15 where they become entraped in the resin coating and deteriorate the coated fiber.
Published European Patent Application EP 0 374 926 teaches that the raw material should be supplied to the reaction chamber under conditions that cause the reaction to proceed more effectively. That publication states: (a) when the reaction chamber diameter is too small, carbon soot is deposited on a surface of an inner wall of the reaction chamber so that a long length of fiber cannot be coated with good quality carbon, and (b) the gaseous raw material does not flow properly when the diameter of the reaction chamber chamber diameter is too large. It concludes that the diameter of the reaction chamber should therefore be at least 2.5 cm and preferably not larger than about 4 cm.
However, when the reaction chamber is designed in accordance with the present invention, substantially no carbon soot deposits on the inner wall when the inside diameter (ID) is about 1 cm or less. Indeed, reaction chamber diameters greater than about 1 cm generated an excessive amount of fluffy soot that became attached to many surfaces and clogged the reactor.
FIG. 2 shows a prior art reactor of the type disclosed in Japanese published Patent Application 83339-1987. Coating apparatus 30 includes reactant supply tube 31 and a pair of isolation chambers 32 and 33. Reactant gas RG is fed through feed chamber 34 and feed holes 35 to tube 31. Most of the length of tube 31 above feed holes 35 is surrounded by an evacuated chamber 36 which prevents the loss of heat. Reaction products flow through exhaust chamber 37 and outlet pipe 38.
As fiber 50 is drawn through apparatus 30, it pulls ambient air through openings 41 and 42 into region 43 located between tip 44 of tube 31 and opening 42. The intake of air through opening 41 is counteracted by liquid isolation chamber 33 which includes an intake opening 40 for supplying a liquid thereto. The liquid can be an abrasion resistant coating material in which case a coolant gas such as helium is flowed into pipe 47 to cool the fiber prior to its entry into the coating material. Chamber 32 is a gas isolation chamber having intake opening 39 for an inert gas such as nitrogen, a portion of which flows from opening 41 and helps to sweep air from fiber 50 as it enters the reactor.
In the apparatus of FIG. 2 reactant gas flows counter to the direction of fiber 50 for the relatively long distance a between holes 35 and tip 44, the gas temperature increasing with further flow. This heating of the reactant gas is enhanced by employing a relatively narrow tube 31, thereby causing the reactant gas to pass near the fiber. In an apparatus in which the distance a was about 16 cm, the diameter of tube 31 was 9 mm, and the diameter of tip 44 was 5 mm, a reaction occurring below tip 44 resulted in a carbonacious buildup that quickly filled tube 31. The type of reaction that occurred in apparatus 30 produced a form of carbon that was loose and fluffy. The formation of this buildup within tube 31 could be substantially eliminated by increasing the combined upward flow of reactant gas and helium to a flow rate sufficient to cause the reaction to move upwardly to region 43. All flows to and from the coating apparatus, i.e. the vacuum applied to pipe 38 and the flows of nitrogen, helium and reactant gas to inlets 39, 47 and 46, respectively, had to be carefully adjusted to optimize the process and cause the reaction to take place in region 43. Increasing reactant gas or helium flow caused the reaction to occur at a higher point. If the point of reaction was too high, carbon deposited on surface 48; if it was too low, carbon deposited on tip 44, such deposits occurring after relatively short runs. This resulted in the plugging of narrow openings through which fiber passes. The buildup finally touched the fiber and caused it to break. Furthermore, the hermetic properties of the fiber were affected by small changes in gas flow rates.
The reactor of FIG. 2 was relatively unstable in that it did not provide a reliable hermetic reaction. The reaction could not be stabilized until about 1 to 2 km of fiber was drawn; this initially produced fiber had to be discarded because of inadequate fatigue protection. The carbon coating was not uniform throughout a run; coating resistivity could be 11 k-ohm per cm length of fiber at the start of a run and 6 k-ohm/cm or less at the end of the run.
In order to produce usable fiber (having an n-value greater than 100 from the apparatus of FIG. 2, the coating resistivity had to be between 6 and 11 k-ohms/cm. Minimum reactant gas flow was required to provide a coating thickness sufficient to achieve the required resistivity. The maximum length of a run (total fiber produced by the apparatus of FIG. 2 before soot fouled the coater) was about 10 km. The average length of a run (total fiber produced before fouling) was 8 km, and the maximum length of usable fiber produced was between 6 and 8 km (some fiber was abraided).
This type of reaction chamber functioned better at low draw speeds because of the lower fiber temperature resulting from such draw speeds. At high draw speeds, i.e. greater than about 1 meter/sec, the fiber was sufficiently hot that it transfered too much heat to the upwardly flowing reactant gas, thereby exacerbating the undesirable buildup problem. The coating deposited at faster draw speeds gave some resistance to hydrogen permeation. The present invention yields fiber with a good hydrogen barrier without the harmful buildup which would terminate the draw process.